It is well known that glass has a theoretical strength which is 500 to 1000 times greater than the design strength at which glass can be safely employed. Glass is an intrinsically strong material with observed strengths under laboratory conditions being reported to be on the order of about 7-14.times.10.sup.9 dynes/cm.sup.2. The theoretical strength of glass and the reported laboratory results have not heretofore had any practical significance for commercial applications of glass. It has been found to be difficult, if not impossible, to manufacture high strength glass on a commercial scale and even more difficult to maintain the glass in a high strength condition.
It is well recognized that the principal cause of the substantial differences between the theoretical strength of glass, the strengths observed under laboratory conditions and the strengths which are actually encountered in commercial production and applications is due to the presence of flaws on the surface of the glass.
Various methods have heretofore been suggested to improve the strength of glass manufactured on a commercial scale. One method was to attempt to manufacture the glass without surface defects. However, because of the inherent problems which are encountered in the production, this suggestion has not proven to be a practical solution.
An additional method which was suggested to improve the strength of glass was to after-treat the surfaces of the glass to remove surface flaws. The methods most commonly employed to remove the surface flaws are to flame polish the surface or to chemically etch the surface.
It was also suggested to subject the surface of the glass to various treatments to increase the compressive strength at the surface. One such surface treatment was to modify the surface of the glass by ion exchange wherein a portion of the smaller ions on or near the surface of the glass were displaced by larger ions.
In another suggested process organic chemical films were applied to the surface of the glass in an attempt to protect the surface of the glass.
Certain of the above processes, for example flame polishing, could, on a temporary basis, significantly increase the compressive strength at the surface of the glass. However, it was found that exposure of the glass to extremely mild conditions such as light abrasion of the glass surface, contact with normal body fluids as by contact with fingers and even mere exposure to normal ambient room moisture would induce surface flaws which reduce the compressive strength at the surface of the glass by orders of magnitude. The organic films applied to the surface of the glass did not adequately protect the glass as they were relatively soft and had poor adhesion to the glass.
The relatively low design strength of glass as compared to its theoretical strength has limited the potential uses of glass. Even in applications where it is highly desirable to use glass because of its unique desirable properties, it is generally necessary to use relatively thick glass to compensate in part for the lack of design strength.